ENDOR characterization of a synthetic diiron hydrazido complex as a model for nitrogenase intermediates

Molybdenum-dependent nitrogenase binds and reduces N₂ at the [Fe₇, Mo, Sg, X, homocitrate] iron-molybdenum cofactor (FeMo-co). Kinetic and spectroscopic studies of nitrogenase variants indicate that a single Fe-S face is the most likely binding site. Recently, substantial progress has been made in determining the structures of nitrogenase intermediates formed during alkyne and N₂ reduction through use of ENDOR spectroscopy. However, constraints derived from ENDOR studies of biomimetic complexes with known structure would powerfully contribute in turning experimentally derived ENDOR parameters into structures for species bound to FeMo-co during N ₂ reduction. The first report of a paramagnetic Fe-S compound that binds reduced forms of N₂ involved Fe complexes stabilized by a bulky β-diketiminate ligand (Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522-4523). Treatment of a sulfidodiiron(II) complex with phenylhydrazine gave an isolable mixed-valence Fe^III-Fe^III complex with a bridging phenylhydrazido (PhNNH₂) ligand, and this species now has been characterized by ENDOR spectroscopy. Using both ¹⁵N, ²H labeled and unlabeled forms of the hydrazido ligand, the hyperfine and quadrupole parameters of the -N-NH₂ moiety have been derived by a procedure that incorporates the (near-) mirror symmetry of the complex and involves a strategy which combines experiment with semiempirical and DFT computations. The results support the use of DFT computations in identifying nitrogenous species bound to FeMo-co of nitrogenase turnover intermediates and indicate that ¹⁴N quadrupole parameters from nitrogenase intermediates will provide a strong indication of the nature of the bound nitrogenous species. Comparison of the large ¹⁴N hyperfine couplings measured here with that of a hydrazine-derived species bound to FeMo-co of a trapped nitrogenase intermediate suggests that the ion(s) are not high spin and/or that the spin coupling coefficients of the coordinating cofactor iron ion(s) in the intermediate are exceptionally small.